The present invention relates to a data interface apparatus and method, particularly but not exclusively for providing an interface for data communications between an analog communications link and a digital communications network.
The use of digital communications networks is becoming widespread, as they provide a number of advantages over analog networks. For example, the Inmarsat-B(trademark) satellite communication system is replacing the Inmarsat-A(trademark) system and provides more efficient use of bandwidth as well as more advanced facilities for voice, data and facsimile communications. In another example, digital terrestrial cellular systems are gradually replacing analog terrestrial cellular systems.
However, much analog communications equipment is still in use and it is not always economical to replace an entire analog system by a digital equivalent.
For example, many large passenger ships are fitted with two-wire communications links connected to a private automatic branch exchange (PABX). It would be desirable to connect such a two-wire system to an Inmarsat-B(trademark) ship earth station (SES), but such equipment typically provides only a digital RS232C port for connection to data terminal equipment (DTE). A local area network may be installed in such ships, but the cost of installing such a network can be prohibitively high.
The document U.S. Pat. No. 5,166,977 discloses an interface for connection between a Group 3 fax machine having an RJ11 connection and a secure telephone unit (STU) having an RS-232 connection.
According to one aspect of the present invention, there is provided an interface which allows an analog communications system to be connected to a digital communications network for which a digital interface is already provided and which converts analog dialling signals from the analog communication system to is digital control codes for output on a digital port to the digital interface. The digital control codes may comprise digital dialling codes and/or codes for controlling the setting of the digital interface. They may for example be AT codes complying with ITU Recommendation V25ter.
According to another aspect of the present invention, there is provided an interface between an analog communications link and a digital communications link, which receives a password encoded as analog dialling signals from the analog communications link and enables connection to the digital communications link if the correct password is received.
According to another aspect of the present invention there is provided a data communications interface which is connected between first and second analog communications sublinks at the remote end of which are connected respectively first and second modems. The interface delays the completion of training on one of the sublinks so as to synchronise completion of training over each of the sublinks. The delaying may be performed by setting a timer from a call setup event over the first sublink and beginning training over the first sublink after a predetermined period has expired.
The interface may be connected to a further interface over a wireless communications sublink, the further interface being connected between the wireless communications sublink and the second communications sublink and operable to send call status information over the wireless sublink. In this case, the communications interface may start training the first modem in response to the start of training of the second modem by the remote interface. Alternatively, or additionally, once training has begun over the first and second sublinks, either interface may delay completion of training with its respective modem by retarding training, for example by not responding to the first transmission of a training signal by the respective modem. In this way, the likelihood of either of first and second terminals connected respectively to the first and second modems timing out because no data is received from the other data terminal after completion of training is reduced.